1. Field of the Invention
The present invention relates to a low speed geared motor using the so-called flexible meshing type planetary gear structure excellent in compactness and heat dissipation.
2. Description of the Prior Art
As reduction gears using a flexible meshing planetary gear structure, there has been known such a type as shown in FIGS. 6 and 7 (Japanese Patent Laid-open No. sho 63-130949).
This reduction gear receives the torque of a motor (not shown) through an input shaft 2, lowers the input shaft speed in a reducing part of a flexible meshing planetary gear structure, and outputs the rotation with a low speed and a large torque from an output shaft 4.
Namely, as the input shaft 2 is rotated, an elliptical eccentric body 8 is rotated through a spline 6. By this rotation, an external gear 12 is rotated through an eccentric body bearing 10 while being deformed in an elliptical shape. The external gear 12 thus deformed meshes with an internal gear 14 at two points in the major axis portion of the external gear 12. Further, since the internal gear 14 is fixed, the external gear 12 is restricted in the free rotation around the axis. As a result, by rotation of the input shaft 2, the above meshing portions are rotated synchronously with the rotation of the input shaft 2.
In this case, the number of teeth in the external gear 12 is set to be smaller than the number of teeth (number of outer pins 14A) in the internal gear 14 by a specified number (2, in this embodiment). Consequently, as the meshing portions are rotated one time by one rotation of the input shaft, the external gear 12 is phase-shifted (rotated around the axis) from the internal gear 14 by a difference in the number of teeth. The rotation around the external gear 12 is transmitted to the output shaft 4 through flange portion 18, while the flexing component (radially deformed component) is absorbed by an annular portion 16 formed integrally with the external gear 12. As a result, when the number of teeth in the external gear is taken as N, a reduction ratio of 2/N can be achieved.
Incidentally, this flexible meshing type planetary gear structure is a power transmission system for transmitting a torque or rotational angle of a motor (in general, an electrically-driven motor or hydraulic motor), and thus never generates power by itself. Namely, in combination with the electrically driven motor, the above planetary gear structure does not function until the electric energy is converted in a torque by the electric motor. Also, in combination with the hydraulic motor, it does not function until the liquid energy is converted into a torque by the hydraulic motor.
In this case, in a process of converting the energy, an energy loss is generated. Further, since the motor itself is rotated at high speed, in the case where the starting, stopping, and/or acceleration/deceleration have to be frequently controlled, an inertia energy stored in the starting or the acceleration tends to be wastefully dissipated as heat through the deceleration or stopping. In this respect, the energy loss is inevitable. Also, as for the control requiring high responsiveness and high positioning accuracy, the inertia energy of the motor rotating at high speed makes it difficult to perform with accurate control.
The motor and the reduction gear, naturally, have to be connected each other using a coupling or a key. This causes backlash in greater or lesser degree between the connected parts. Further, the shaft itself is elastically deformed by the applied torque, and the input shaft, cam, bearing and the like are also elastically deformed, so that hysteresis is generated therein. In the case where both a moving direction and a stopping position have to be controlled, for example, in the revolute joints of a robot, the backlash and the hysteresis make it difficult to perform with accurate control.
To cope with the above problems, there has been proposed such a low speed geared motor as shown in FIGS. 8 and 9. The low speed geared motor mainly includes a stator with 3-phase, 2-pole winding, a flexible rotor 52 which is disposed around the outer periphery of the stator 50 so as to from a magnetic path of a rotational magnetic field and also to be freely deformed in an elliptical shape by the applied magnetic attracting force, external teeth 54 formed around the outer periphery of the flexible rotor 52, and internal teeth 56 disposed around the outer periphery of the external teeth 54. Thus, by electrically rotating the direction of forming the magnetic path of the stator 50, the flexing direction of the flexible rotor 52 is rotated. The meshing positions between the internal teeth 54 and the external teeth 56 are rotated by this rotating flexing direction. Consequently, the rotational component of the flexible rotor 52 is transmitted by the output shaft 62 through a boss 60 provided on the bottom circular portion 58 of the flexible rotor 52. The reduction ratio generated depends on the difference in number of teeth between the internal teeth 56 and the external teeth 54.
This low speed geared motor is not of a type in which the high-speed rotation of a motor is reduced by a reduction gear part to be outputted, but of a type in which the flexible deformation due to a magnetic force is applied to the above-described flexible meshing type planetary gear structure. Therefore, it has a possibility of solving the various problems described above.
However, the low speed geared motor described above has such a disadvantage that, since the stator acting as a heat generating source is positioned at the innermost periphery, the motor is not good for the heat dissipation and is not suitable for long operation. Also, since the stator is constituted of the windings using wires and is disposed in the inside, the central portion of the stator cannot be used as a through hole. This makes it impossible to mount the above motor from both sides by passing an input shaft of a mating member through the central portion of the stator, or to use the central portion as a space through which a cable for transmitting a power or electric signals passes.
Also, since the attracting force for flexing the flexible rotor is structurally generated only by a magnetic field formed by the stator, it is not so large. Accordingly, for keeping a large attracting force, it is required to carefully select the materials of the stator and the flexible rotor, to enlarge the stator and the flexible rotor, and to increase a current applied to the stator. This brings about problems of increasing cost, weight, and dimensions, or of causing the increased power consumption and temperature rise.